The LM393 speed sensor is a module made of a fork (U) shaped optical sensor, with infrared light passing between its two arms, and an LM393 comparator chip. A slotted encoder disk sits in the gap: as the disk turns, light passes through a slot and is blocked again, and the D0 output produces a pulse. On a 20-slot disk, 20 pulses mean one full turn. Counting pulses over a set time gives the motor speed (RPM), the wheel speed or the distance a robot has travelled. It runs on 3.3–5 V and connects to a digital pin on the Arduino, ESP32 or Pico. It is best read with an interrupt (attachInterrupt) so no pulses are missed. It is often used as a wheel encoder on robot cars with geared DC motors.